Sensing circuit and method for a capacitive touch panel

ABSTRACT

A sensing method and circuit for a capacitive touch panel sense the capacitance variation of a lateral capacitor formed at the intersection of two traces of the capacitive touch panel, to distinguish a real point from a ghost point. A sensing cycle includes two non-overlapping clock phases. In the first clock phase, the voltages across the lateral capacitor and across a sensing capacitor are set. In the second clock phase, the voltage at a first, terminal of the lateral capacitor is changed, and a second terminal of the lateral capacitor is connected to a first terminal of the sensing capacitor, causing a voltage variation at a second terminal of the sensing capacitor. This voltage variation is used to determine whether the intersection is touched. The sensing method and circuit reflect the status of the lateral capacitor in real-time and prevent the location of the touch point from being misjudged.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of co-pending application Ser. No.12/652,260, filed on Jan. 5, 2010, for which priority is claimed under35 U.S.C. §120; and this application claims priority of Application No.098100727 filed in Taiwan on Jan. 9, 2009 under 35 U.S.C. §119; andApplication No. 098117362 filed in Taiwan on May 25, 2009 under 35U.S.C. §119, the entire contents of all of which are hereby incorporatedby reference.

FIELD OF THE INVENTION

The present invention is related generally to a capacitive touch paneland, more particularly, to a sensing circuit and method for a capacitivetouch panel.

BACKGROUND OF THE INVENTION

As shown in FIG. 1, an XY-programmed capacitive touch panel 10 includesa plurality of X-direction traces TX1-TX8 and a plurality of Y-directiontraces TY1-TY6, whose positioning method includes scanning theX-direction traces TX1-TX8 and the Y-direction traces TY1-TY6 to locatethe touch point according to the capacitance variations in direction andY-direction. For example, when a finger touches a point 12 on thecapacitive touch panel 10, the capacitance values of the traces TXT andTY3 are changed, so it can be determined that the finger is at theintersection 12 of the traces TX8 and TY3. However, for multi-touchapplications, this positioning method is unable to correctly identifythe touch points. Taking a two-finger application as shown in FIG. 2 foran example, two fingers touching the capacitive touch panel 10simultaneously at touch points 20 and 22 respectively, will cause thecapacitance values of the traces TX2, TX4, TY2, and TY4 changed. In thiscase, there are two possible pairs of touch points according to thecapacitance variations, in addition to the real touch points 20 and 22,i.e., the positions (TX2, TY4) and (TX4, TY2) where the fingers actuallytouch, two ghost points 24 and 26, i.e., the positions (TX2, TY2) and(TX4, TY4), are present, which will make the capacitive touch panel 10unable to accurately identify the real touch points 20 and 22.

Therefore, it is desired a solution for a capacitive touch panel todistinguish real points from ghost points.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to provide a sensing method andcircuit for a capacitive touch panel.

Another object of the present invention is to provide a method andcircuit for a capacitive touch panel to distinguish real points fromghost points.

According to the present invention, a sensing method for a capacitivetouch panel includes applying a first voltage to a first trace and asecond trace of the capacitive touch panel and setting the voltageacross a sensing capacitor in a first clock phase, and switching thefirst trace being connected from the first voltage to a second voltageand connecting the second trace to a first terminal of the sensingcapacitor in a second clock phase, thereby causing a voltage variationat a second terminal of the sensing capacitor.

According to the present invention, a sensing circuit for a capacitivetouch panel includes a first switching circuit connected to a firsttrace of the capacitive touch panel to connect the first trace to afirst voltage terminal in a first clock phase and to a second voltageterminal in a second clock phase, an operational amplifier having afirst input connected to the first voltage terminal, a second input, andan output, a second switching circuit connected to a second trace of thecapacitive touch panel to connect the second trace to the first voltageterminal in the first clock phase and to the second input of theoperational amplifier in the second clock phase, a sensing capacitorhaving a first terminal connected to the second input of the operationalamplifier, and a second terminal, a third switching circuit connectedbetween the second input and the output of the operational amplifier toconnect the output of the operational amplifier to the second input inthe first clock phase, and a fourth switching circuit connected to thesecond terminal of the sensing capacitor to connect the second terminalof the sensing capacitor to the first voltage terminal in the firstclock phase and to the output of the operational amplifier in the secondclock phase.

According to the present invention, a sensing method for a capacitivetouch panel includes in a first clock phase, applying a first voltageand a second voltage to a first trace and a second trace of thecapacitive touch panel, respectively, and setting the voltage across asensing capacitor, and in a second clock phase, switching, the firsttrace being connected from the first voltage to a third voltage andconnecting the second trace to a first terminal of the sensingcapacitor, thereby causing a voltage variation at a second terminal ofthe sensing capacitor.

According to the present invention, a sensing circuit for a capacitivetouch panel includes a first switching circuit connected to a firsttrace of the capacitive touch panel to connect the first trace to afirst voltage terminal in a first clock phase and to a second voltageterminal in a second clock phase, an operational amplifier having afirst input connected to the second voltage terminal, a second input,and an output, a second switching circuit connected to a second trace ofthe capacitive touch panel to connect the second trace to the secondvoltage terminal in the first dock phase and to the second input of theoperational amplifier in the second clock phase, a sensing capacitorhaving a first terminal connected to the second input of the operationalamplifier, and a second terminal, a third switching circuit connectedbetween the second input and the output of the operational amplifier toconnect the output of the operational amplifier to the second input inthe first clock phase, and a fourth switching circuit connected to thesecond terminal of the sensing capacitor and configured to connect thesecond terminal of the sensing capacitor to the second voltage terminalin the first clock phase and to the output of the operational amplifierin the second clock phase.

The sensing method and circuit according to the present invention aredesigned to sense the capacitance variation of a lateral capacitorformed at the intersection of two traces of a capacitive touch panel, soas to distinguish from real points and ghost points when the capacitivetouch panel is touched.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and advantages of the presentinvention will become apparent to those skilled in the art uponconsideration of the following description of the preferred embodimentsaccording to the present invention taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is a schematic diagram showing a conventional capacitive touchpanel;

FIG. 2 is a schematic diagram showing a two-finger application;

FIG. 3 is a schematic diagram showing the principle on which the presentinvention is based;

FIG. 4 is a flowchart of a sensing method according to the presentinvention;

FIG. 5 is a circuit diagram of an embodiment for the sensing circuitaccording to the present invention;

FIG. 6 is the circuit diagram of FIG. 5 when sensing a real point;

FIG. 7 is an equivalent circuit of FIG. 6 in a first clock phase;

FIG. 8 is an equivalent circuit of Fig, 6 in a second clock phase;

FIG. 9 is the circuit diagram of Fig, 5 when sensing a ghost point;

FIG. 10 is an equivalent circuit of FIG.) in a first clock phase;

FIG. 11 is an equivalent circuit of FIG. 9 in a second clock phase;

FIG. 12 is a circuit diagram of a first embodiment for a determinationcircuit;

FIG. 13 is a circuit diagram of a second embodiment for a determinationcircuit;

FIG. 14 is a circuit diagram of a second embodiment for a sensingcircuit according to the present invention;

FIG. 15 is a circuit diagram of FIG. 14 when sensing a real point;

FIG. 16 is an equivalent circuit of FIG. 15 in a first clock phase;

FIG. 17 is an equivalent circuit of FIG. 15 in a second clock phase;

FIG. 18 is a circuit diagram of FIG. 14 when sensing a ghost point;

FIG. 19 is an equivalent circuit of FIG. 1$ in a first clock phase and

FIG. 20 is an equivalent circuit of FIG. 18 in a second clock phase.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 3 is a schematic diagram showing the principle on which the presentinvention is based. At the intersection of two traces TXN and TYM of acapacitive touch panel, there will be a parasitic lateral capacitor 30is formed between the two traces TXN and TYM, whose capacitance value isrepresented by Cxy. When a finger touches at the intersection of thetraces TXN and TYM, not only are the capacitance values of the tracesTXN and TYM changed, but also is the capacitance value Cxy of thelateral capacitor 30 changed. The capacitance variation of the lateralcapacitor 30 can be used to identify a real point on the capacitivetouch panel. For example, responsive to a two-finger touch as shown inFIG. 2, the lateral capacitance values of the real point (TX2, TY4) and(TM, TY2) are changed, whereas the lateral capacitance values of theghost points (TX2, TY2) and (TX4, TY4) are not. Therefore, using asensing circuit 40 to sense the lateral capacitance values candistinguish the real points from the ghost points according to thecapacitance variations of the lateral capacitors.

FIG. 4 is a flowchart of a sensing method according to the presentinvention, in which a sensing cycle of the sensing circuit 40 forsensing the lateral capacitor 30 includes two clock phases. At step 90,in the first clock phase, the sensing circuit 40 applies a same voltageto the two traces TXN and TYM, and sets the voltage across a sensingcapacitor thereof. To set the voltage across the sensing, capacitor, itmay apply a same voltage or to different voltages to the two terminalsof the sensing capacitor. For instance, the voltage applied to thetraces TXN and TYM is also applied to the two terminals of the sensingcapacitor. At step 92, i.e., in the second clock phase, the sensingcircuit 40 changes the voltage of the trace TXN and connects the traceTYM to the first terminal of the sensing capacitor to cause the voltageat the second terminal of the sensing capacitor changed. This voltagevariation at the second terminal of the sensing capacitor is related tothe current capacitance value of the lateral capacitor 30 and can beused to determine whether the intersection is touched, therebydistinguishing a real point from a ghost point.

FIG. 5 is a circuit diagram of an embodiment for the sensing circuit 40,in which the traces TXN and TYM of FIG. 3 are shown as an equivalentcircuit 50, the trace TXN includes a self capacitor 5002 whosecapacitance value is Cx, the trace TYM includes a self capacitor 5004whose capacitance value is Cy, and the lateral capacitor 30 between thetraces TXN and TYM has a capacitance value Cxy. The sensing circuit 40is connected to the traces TXN and TYM and senses the capacitancevariation of the lateral capacitor 30 to thereby determine whether theintersection of the traces TXN and TYM is touched by a finger. In thesensing circuit 40, a switching circuit 4002 includes a switch SW1connected between a voltage terminal Vc and the trace TXN, and a switchSW2 connected between the trace TXN and a voltage terminal Vcom, and theswitches SW1 and SW2 are controlled by clock phases P2 and P1,respectively. A switching circuit 4004 includes a switch SW3 connectedbetween the trace TYM and the voltage terminal Vcom, and a switch SW4connected between the trace TYM and an input 4012 of an operationalamplifier 4010, and the switches SW3 and SW4 are controlled by the clockphases P1 and P2, respectively. The operational amplifier 4010 hasanother input 4014 connected to the voltage terminal Vcom. A switchingcircuit 4006 includes a switch SW5 connected between the input 4012 andan output 4016 of the operational amplifier 4010 and controlled by theclock phase P1. A sensing capacitor CF has a first terminal 4018connected to the input 4012 of the operational amplifier 4010, and asecond terminal 4020 connected to a switching circuit 4008. Theswitching circuit 4008 includes a switch SW6 connected between thesecond terminal 4020 of the sensing capacitor CF and the voltageterminal Vcom, and a switch SW7 connected between the second terminal4020 of the sensing capacitor CF and the output 4016 of the operationalamplifier 4010, and the switches SW6 and SW7 are controlled by the clockphases P1 and P2, respectively. The clock phases P1 and P2 do notoverlap with each other.

FIG. 6 is the circuit diagram of FIG. 5 when a finger touches theintersection of the traces TXN and TYM, in which the self capacitors5002 and 5004 of the traces TXN and TYM have increases in capacitanceΔCx and ΔCy, respectively, and the lateral capacitor 30 has an increasein capacitance ΔCxy. FIGS. 7 and FIGS. 8 are equivalent circuits of thesensing circuit 40 in the clock phases P1 and P2, respectively.Referring to FIG. 7, in the clock phase P1, the switches SW2, SW3, SW5and SW6 are closed, and the switches SW1, SW4 and SW7 are opened.Consequently, the traces TXN and TYM are both connected to the voltageterminal Vcom, the second terminal 4020 of the sensing capacitor CF isconnected to the voltage terminal Vcom, and the output 4016 of theoperational amplifier 4010 is connected to the input 4012 of theoperational amplifier 4010. As the self capacitor 5002 of the trace TXNhas the capacitance increase ΔCx, the charge thereof isQcx=Vcom×(Cx+ΔCx).   [Eq-1]Similarly, as the self capacitor 5004 of the trace TYM has thecapacitance increase ΔCy, the charge thereof isQcy=Vcom×(Cy+ΔCy).   [Eq-2]Since the two terminals of the lateral capacitor 30 are equal involtage, the total charge of the lateral capacitor 30 is zero. Inaddition, due to the virtual short circuit, the voltage at the input4012 of the operational amplifier 4010 is equal to the voltage Vcom atthe input 4014. Therefore, the two terminals 4018 and 4020 of thesensing capacitor CF are equal in voltage, which leads to zero charge ofthe sensing capacitor CF, and the voltage Vo at the output 4016 of theoperational amplifier 4010 is equal to Vcom. Afterward, in the clockphase P2, as shown in FIG. 8, the switches SW2, SW3, SW5 and SW6 areopened, and the switches SW1, SW4 and SW7 are closed. In consequence,the trace TXN is connected to the voltage terminal Vc, the trace TYM isconnected to the input 4012 of the operational amplifier 4010, and thesecond terminal 4020 of the sensing capacitor CF is connected to theoutput 4016 of the operational amplifier 4010. Meanwhile, the output4016 and the input 4012 of the operational amplifier 4010 aredisconnected from each other. Under this circumstance, the charge of theself capacitor 5002 of the trace TXN isQcx=Vcx(Cx+ΔCx),   [Eq-3]and the charge of the self capacitor 5004 of the trace TYM isQcy=Vcom×(Cy+ΔCy).   [Eq-4]Now that the TXN-side voltage of the lateral capacitor 30 has beenswitched from Vcom to Vc, and the TYM-side terminal of the lateralcapacitor 30 is connected to the first terminal 4018 of the sensingcapacitor CF, the charge of the lateral capacitor 30 isQcxy=(Vc−Vcom)×(Cxy+ΔCxy).   [Eq-5]Due to conservation of charge, a voltage variation occurs at the secondterminal 4020 of the sensing capacitor CF. According to the equationEq-5, the charge of the sensing capacitor CF isQcf=(Vo−Vcom)×CF=−(Vc−Vcom)×(Cxy+ΔCxy).  [Eq-6]From the equation Eq-6, the voltage at the output 4016 of theoperational amplifier 4010 isVo=[−(Cxy−ΔCxy)/CF]×(Vc−Vcom)+Vcom.   [Eq-7]It can be known from the equation Eq-7 that the capacitance variationsΔCx and ΔCy of the self capacitors 5002 and 5004 of the traces TXN andTYM do not affect the output Vo of the sensing circuit 40.

FIG. 9 is a schematic diagram showing how a ghost point is sensed, inwhich the self capacitors 5002 and 5004 of the traces TXN and TYM havethe capacitance increases ΔCx and ΔCy, respectively, but the lateralcapacitor 30 has no increase in capacitance because the intersection ofthe traces TXN and TYM is not touched. FIGS. 10 and 11 are equivalentcircuits of the sensing circuit 40 in the clock phases P1 and P2,respectively. In the clock phase P1, as shown in FIG. 10, the switchesSW2, SW3, SW5 and SW6 are closed, and the switches SW1, SW4 and SW7 areopened. Hence, the traces TXN and TYM are both connected to the voltageterminal Vcom, and the second terminal 4020 of the sensing capacitor CFis connected to the voltage terminal Vcom, and the output 4016 and theinput 4012 of the operational amplifier 4010 are connected together.Since the self capacitor 5002 of the trace TXN has the capacitanceincrease ΔCx, the charge thereof is expressed by the equation Eq-1.Similarly, as the self capacitor 5004 of the trace TYM has thecapacitance increase ΔCy, the charge thereof is expressed by theequation Eq-2. On the other hand, the charge of the lateral capacitor 30is zero because the two terminals of the lateral capacitor 30 are equalin voltage Due to virtual short circuit, the voltage at the input 4012of the operational amplifier 4010 is equal to the voltage Vcom at theinput 4014. As a result, the two terminals 4018 and 4020 of the sensingcapacitor CF are equal in voltage, the charge of the sensing capacitorCF is zero, and the voltage Vo at the output 4016 of the operationalamplifier 4010 is equal to Vcom. In the subsequent clock phase P2,referring to FIG. 11, the switches SW2, SW3, SW5 and SW6 are opened, andthe switches SW1, SW4 and SW7 are closed. Thus, the trace TXN isconnected to the voltage terminal Vc, the trace TYM is connected to theinput 4012 of the operational amplifier 4010, and the second terminal4020 of the sensing capacitor CF is connected to the output 4016 of theoperational amplifier 4010, and the output 4016 and the input 4012 ofthe operational amplifier 4010 are disconnected from each other. Underthis circumstance, the charge of the self capacitor 5002 of the traceTXN is expressed by the equation Eq-3, and the charge of the selfcapacitor 5004 of the trace TYM is expressed by the equation Eq-4. Sincethe TXN-side voltage of the lateral capacitor 30 has been switched fromVcom to Vc, and the TYM-side terminal of the lateral capacitor 30 isconnected to the first terminal 4018 of the sensing capacitor CF, thecharge of the lateral capacitor 30 isQcxy=(Vc−Vcom)×Cxy.   [Eq-8]Due to conservation of charge, the voltage at the second terminal 4020of the sensing capacitor CF is changed. According to the equation Eq-8,the charge of the sensing capacitor CF isQcf=(Vo−Vcom)×C=−(Vc−Vcom)×Cxy.   [Eq-9]Further, the voltage at the output 4016 of the operational amplifier4010 is derived from the equation Eq-9 asVo=(−Cxy/CF) ×(Vc−Vcom)+Vcom.   [Eq-10]It can be known from the equation Eq-10 that the capacitance variationsΔCx and ΔCy of the self capacitors 5002 and 5004 of the traces TXN andTYM do not affect the output Vo of the sensing circuit 40. Besides, acomparison between the equations Eq-7 and Eq-10 shows that, as thelateral capacitor 30 of a real point differs from that of a ghost pointin capacitance values, the voltage Vo at the output 4016 of theoperational amplifier 4010 varies accordingly. Hence, the magnitude ofthe voltage Vo can be used to determine whether the intersection of thetraces TXN and TYM is touched by a finger. For example, now that thedifference between the voltage Vo of a real point and that of a ghostpoint is ΔCxy(Vc−Vcom)/CF, if the voltage Vo of a point being detectedexceeds a certain threshold value, it can be determined that the pointin question is a real point. FIG. 12 is a circuit diagram of anembodiment for the determination circuit, in which a comparator 4030compares the voltage Vo with a threshold voltage Vth in order todetermine whether the intersection of the traces TXN and TYM is touched.For example, a ghost point is identified if the signal GP is logic high(1), and a real point is identified if the signal GP is logic low (0).The value of the threshold voltage Vth determines the sensitivity of thecircuit toward the capacitance variation ΔCxy of the lateral capacitor30. Viewing from another perspective, the relationship of Vo=Vcom existsin the first clock phase P1 regardless of whether the pointed beingdetected is a real point or a ghost point. In the second clock phase P2,however, the voltage Vo of a real point is different from that of aghost point. Therefore, it is also feasible to distinguish a real pointfrom a ghost point by the variation of Vo in the two clock phases P1 andP2. Referring to FIG. 13 for another embodiment of the determinationcircuit, a differential amplifier 4032 with an amplification coefficientK has two inputs for receiving the voltages Vo and Vcom, respectively,and an output from the differential amplifier 4032 is sent to acomparator 3030 along with the threshold voltage Vth. The point beingdetected is identified as a ghost point if the signal GP generated islogic high (1), and as a real point if the signal GP is logic low (0).The values of the amplification coefficient K and the threshold voltageVth determine the sensitivity of the circuit toward the capacitancevariation ΔCxv of the lateral capacitor 30.

In the embodiments shown in FIGS. 6 through 11, a single sensing cycleincludes two non-overlapping clock phases P1 and P2. The operation inthe clock phase P1 involves resetting the voltages across the lateralcapacitor 30 and across the sensing capacitor CF to zero such that thecharges of the lateral capacitor 30 and of the sensing capacitor CF rezero. In another embodiment, however, the voltage across the lateralcapacitor 30 can be set at a value other than zero in the clock phaseP1. In the clock phase P2 that follows, the TXN-side voltage of thelateral capacitor 30 is changed, and due to conservation of charge, thevoltage at the output Vo of the sensing circuit 40 is changed at thesame time. Thus, the present status of the lateral capacitor 30 isreflected in real time at the output Vo of the sensing circuit 40,allowing the position of a real point to be determined accurately.

When the sensing circuit 40 is applied to the capacitive touch panel 10of FIG. 1, and the capacitive touch panel 10 is simultaneously touchedby two fingers at the points 20 and 22 as shown in FIG. 2, the selfcapacitances of all the traces TX2, TX4, TY2, and TY4 are changed.However, since points 24 and 26 are not touched by the fingers, theintersection of the traces TX2 and TY2 shows no lateral capacitancevariation, nor does the intersection of the traces TX4 and TY4. Hence,the possibility of the real points being located at the points 24 and 26can be eliminated, thereby preventing the errors which may otherwiseresult from. the ghost points.

FIG. 14 is a circuit diagram of another embodiment for the sensingcircuit 40. The traces TXN and TYM of FIG. 3 are depicted in FIG. 14 asan equivalent circuit 50, in which the trace TXN has a self capacitor5002 with a capacitance value Cx, the trace TYM has a self capacitor5004 with a capacitance value Cy, and the lateral capacitor 30 betweenthe traces TXN and TYM has a capacitance value Cxy. In the sensingcircuit 40, a switching circuit 4002 includes a switch SW1 connectedbetween a voltage terminal Vc and the trace TXN, and a switch SW2connected between the trace TXN and a voltage terminal Vcom, and theswitches SW1 and SW2 are controlled by the clock phases P1 and P2,respectively. A switching circuit 4004 includes a switch SW3 connectedbetween the trace TYM and the voltage terminal Vcom, and a switch SW4connected between the trace TYM and an input 4012 of an operationalamplifier 4010, and the switches SW3 and SW4 are controlled by the clockphases P1 and P2, respectively. The operational amplifier 4010 hasanother input 4014 connected to the voltage terminal Vcom. A switchingcircuit 4006 includes a switch SW5 connected between the input 4012 andan output 4016 of the operational amplifier 4010 and controlled by theclock phase P1. A sensing capacitor CF has a first terminal 4018 and asecond terminal 4020, of which the first terminal 4018 is connected tothe input 4012 of the operational amplifier 4010, and the secondterminal 4020 is connected to a switching circuit 4008. The switchingcircuit 4008 includes a switch SW6 connected between the second terminal4020 of the sensing capacitor CF and the voltage terminal Vcom, and aswitch SW7 connected between the second terminal 4020 of the sensingcapacitor CF and the output 4016 of the operational amplifier 4010, ofwhich the switches SW6 and SW7 are controlled by the clock phases P1 andP2, respectively.

FIG. 15 is the circuit diagram of FIG. 14 when a finger touches theintersection of the traces TXN and TYM, in which the self capacitors5002 and 5004 of the traces TXN and TYM have increases in capacitanceΔCx and ΔCy, respectively, and the lateral capacitor 30 has an increasein capacitance ΔCxy. FIGS. 16 and 17 are equivalent circuits of thesensing circuit 40 depicted in FIG. 15 in the clock phases P1 and P2,respectively. Referring to FIG. 16, in the clock phase P1, the switchesSW1, SW3, SW5 and SW6 are closed, and the switches SW2, SW4, and SW7 areopened. Consequently, the trace TXN is connected to the voltage terminalVc, the trace TYM is connected to the voltage terminal Vcom, and thesecond terminal 4020 of the sensing capacitor CF is connected to thevoltage terminal Vcom, while the output 4016 and the input 4012 of theoperational amplifier 4010 are connected together. Since the selfcapacitor 5002 of the trace TXN has the capacitance increase ΔCx, thecharge thereof isQcx=Vc×(Cx+ΔCx).   [Eq-11 ]

Similarly, as the self capacitor 5004 of the trace TYM has thecapacitance increase ΔCy, the charge thereof isQcy=Vcom×(Cy+ΔCy).   [Eq-12]By the same token, the charge of the lateral capacitor 30 isQcxy=(Vc−Vcom)×(Cxy+ΔCxy).   [Eq-13]Due to virtual short circuit, the voltage at the input 4012 of theoperational amplifier 4010 is equal to the voltage Vcom at the input4014, Therefore, the terminals 4018 and 4020 of the sensing capacitor CFare equal in voltage, and the charge of the sensing capacitor CF iszero. Meanwhile, the voltage Vo at the output 4016 of the operationalamplifier 4010 is equal to Vcom. In the subsequent clock phase P2, asshown in FIG. 17, the switches SW1, SW3, SW5 and SW6 are opened, and theswitches SW2, SW4 and SW7 are closed. In consequence, the trace TXN isconnected to the voltage terminal Vcom, the trace TYM is connected tothe input 4012 of the operational amplifier 4010, and the secondterminal 4020 of the sensing capacitor CF is connected to the output4016 of the operational amplifier 4010. Meanwhile, the output 4016 andthe input 4012 of the operational amplifier 4010 are disconnected fromeach other. Under this circumstance, the charge of the self capacitor5002 of the trace TXN isQcx=Vcom×(Cx+ΔCx),   [Eq-14]and the charge of the self capacitor 5004 of the trace TYM isQcy=Vcom×(Cy+ΔCy).   [Eq-15]Now that the two terminals of the lateral capacitor 30 are equal involtage, the charge of the lateral capacitor 30 is zero. Due toconservation of charge, the voltage at the second terminal 4020 of thesensing capacitor CF is changed. According to the equation Eq-13,thecharge of the sensing capacitor CF isOcf=(Vo−Vcom)×CF=(Vc−Vcom)×(Cxy +ΔCxy).   [Eq-16]From the equation Eq-16, the voltage at the output 4016 of theoperational amplifier 4010 is derived asVo=[(Cxy+ΔCxy)/CF]×(Vc−Vcom)+Vcom.   [Eq-17]It can be known from the equation Eq. 17 that the capacitance variationsΔCx and ΔCy of the self capacitors 5002 and 5004 of the traces TXN andTYM do not affect the output Vo of the sensing circuit 40.

FIG. 18 is the circuit diagram of FIG. 14 when sensing a ghost point.While the self capacitors 5002 and 5004 of the traces TXN and TYM havethe capacitance increases ΔCx and ΔCy, respectively, the lateralcapacitor 30 has no capacitance increase because the intersection of thetraces TXN and TYM is not touched. FIGS. 19 and 20 are equivalentcircuits of the sensing circuit 40 depicted in FIG. 18 in the clockphases P1 and P2, respectively. In the clock phase P1, as shown in FIG.19, the switches SW1, SW3, SW5 and SW6 are closed, and the switches SW2,SW4 and SW7 are opened. Hence, the trace TXN is connected to the voltageterminal Vc, the trace TYM is connected to the voltage terminal Vcom,and the second terminal 4020 of the sensing capacitor CF is connected tothe voltage terminal Vcom, while the output 4016 and the input 4012 ofthe operational amplifier 4010 are connected together. Since the selfcapacitor 5002 of the trace TXN has the capacitance increase ΔCx, thecharge thereof is expressed by the equation Eq-11. Similarly, as theself capacitor 5004 of the trace TYM has the capacitance increase ΔCy,the charge thereof is expressed by the equation Eq-12. On the otherhand, the charge of the lateral capacitor 30 isQcxy=(Vc−Vcom)×Cxy.   [Eq-18]Due to virtual short circuit, the voltage at the input 4012 of theoperational amplifier 4010 is equal to the voltage Vcom at the input4014. As a result, the two terminals 4018 and 4020 of the sensingcapacitor CF are equal in voltage, and therefore the charge of thesensing capacitor CF is zero. Meanwhile, the voltage Vo at the output4016 of the operational amplifier 4010 is equal to Vcom. In thesubsequent clock phase P2, referring to FIG. 20, the switches SW1, SW3,SW5 and SW6 are opened, and the switches SW2, SW4 and SW7 are closed.Therefore, the trace TXN is connected to the voltage terminal Vcom, thetrace TYM is connected to the input 4012 of the operational amplifier4010, and the second terminal 4020 of the sensing capacitor CF isconnected to the output 4016 of the operational amplifier 4010, whilethe output 4016 and the input 4012 of the operational amplifier 4010 aredisconnected from each other. Under this circumstance, the charge of theself capacitor 5002 of the trace TXN is expressed b the equation Eq-14,and the charge of the self capacitor 5004 of the trace TYM is expressedby the equation Eq-15. Since the two terminals of the lateral capacitor30 are equal in voltage., the charge of the lateral capacitor 30 iszero, and due to conservation of charge, the voltage at the secondterminal 4020 of the sensing capacitor CF is changed. According to theequation Eq-18, the charge of the sensing capacitor CF isQcf=(Vo−Vcom)×CF=(Vc−Vcom)×Cxy.   [Eq-19]From the equation Eq-19, the voltage at the output 4016 of theoperational amplifier 4010 is obtained asVo=(Cxy/CF)×(Vc−Vcom)+Vcom.   [Eq-20]

It can be known from the equations Eq-17 and Eq-20 that, as the lateralcapacitor 30 of a real point differs from that of a ghost point incapacitance value, the voltage Vo at the output 4016 of the operationalamplifier 4010 varies accordingly. Hence, the magnitude of the voltageVo can be used to determine whether the intersection of the traces TXNand TYM is touched by a finger.

While the present invention has been described in conjunction withpreferred embodiment thereof, it is evident that many alternatives,modifications and variations will be apparent to those skilled in theart. Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and scopethereof as set forth in the appended claims.

What is claimed is:
 1. A sensing method for a capacitive touch panelincluding a first trace, a second trace, and a lateral capacitor formedat an intersection of the first trace and the second trace, the sensingmethod comprising: (a) in a first clock phase, applying a first voltageto the first trace and the second trace and setting a voltage across asensing capacitor, wherein during the first clock phase, the first traceis always set to the first voltage; and (b) in a second clock phase,switching the first trace being connected from the first voltage to asecond voltage and connecting the second trace to a first terminal ofthe sensing capacitor to thereby cause a voltage variation at a secondterminal of the sensing capacitor; wherein the second terminal of thesensing capacitor is connected to a first input terminal of anoperational amplifier and the first terminal of the sensing capacitor isconnected to a second input terminal of the operational amplifier duringthe first clock phase; and wherein the first terminal of the sensingcapacitor is connected to both the second input terminal of theoperational amplifier and the second trace, and the second terminal ofthe sensing capacitor is connected to an output terminal of theoperational amplifier during the second clock phase.
 2. The sensingmethod of claim 1, wherein the step a comprise applying the thirdvoltage to the first terminal and the second terminal of the sensingcapacitor.
 3. The sensing method of claim 2, wherein the third voltageis equal to the first voltage.
 4. The sensing method of claim 2, whereinthe third voltage is different from the first voltage and the secondvoltage.
 5. The sensing method of claim 1, wherein the step b comprises:applying the first voltage to a first input of the operationalamplifier; connecting the second trace as well as the first terminal ofthe sensing capacitor to a second input of the operational amplifier;and connecting the second terminal of the sensing capacitor to theoutput of the operational amplifier.
 6. The sensing method of claim 1,further comprising determining whether the intersection is touchedaccording to the voltage variation at the second terminal of the sensingcapacitor.
 7. A sensing circuit for a capacitive touch panel including afirst trace, a second trace, and a lateral capacitor formed at anintersection of the first trace and the second trace, the sensingcircuit comprising: a first switching circuit connected to the firsttrace to connect the first trace to a first voltage terminal in a firstclock phase and to a second voltage terminal in a second clock phase; anoperational amplifier having a first input connected to the firstvoltage terminal, a second input, and an output; a second switchingcircuit connected to the second trace to connect the second trace to thefirst voltage terminal in the first clock phase and to the second inputof the operational amplifier in the second clock phase; a sensingcapacitor having a first terminal connected to the second input of theoperational amplifier, and a second terminal; a third switching circuitconnected between the second input and the output of the operationalamplifier to connect the output of the operational amplifier to thesecond input in the first clock phase; and a fourth switching circuitconnected to the second terminal of the sensing capacitor to connect thesecond terminal of the sensing capacitor to the first input of theoperational amplifier in the first clock phase and to the output of theoperational amplifier in the second clock phase; wherein the fourthswitching circuit comprises: a first switch for connecting the secondterminal of the sensing capacitor to the first input of the operationalamplifier in the first clock phase; and a second switch for connectingthe second terminal of the sensing capacitor to the output of theoperational amplifier in the second clock phase; wherein the firstterminal of the sensing capacitor is connected to the second input ofthe operational amplifier and the second terminal of the sensingcapacitor is connected to the first input of the operational amplifierduring the first clock phase; and wherein the first terminal of thesensing capacitor is connected to both the second input of the operationamplifier and the second trace, and the second terminal of the sensingcapacitor is connected to the output of the operational amplifier duringthe second clock phase.
 8. The sensing circuit of claim 7 wherein thefirst switching circuit comprises: a third switch connected between thefirst trace and the second voltage terminal; and a fourth switchconnected between the first trace and the first voltage terminal.
 9. Thesensing circuit of claim 7, wherein the second switching circuitcomprises: a third switch connected between the second trace and thefirst voltage terminal; and a fourth switch connected between the secondtrace and the. second input of the operational amplifier.
 10. Thesensing circuit of claim 7, wherein the third switching circuitcomprises a switch connected between the second input and the output ofthe operational amplifier.
 11. The sensing circuit of claim 7, furthercomprising a comparator connected to the output of the operationalamplifier to compare the voltage at the output with a threshold value inthe second clock phase, so as to determine whether the intersection istouched.
 12. The sensing circuit of claim 7, further comprising: adifferential amplifier connected to the first voltage terminal and theoutput of the operational amplifier to amplify a voltage differencetherebetween; and a comparator connected to the differential amplifierto compare the amplified voltage difference with a threshold value inthe second clock phase, so as to determine whether the intersection istouched.
 13. The sensing circuit according to claim 7, wherein duringthe first clock phase, the first trace is always connected to the firstvoltage terminal.